History of Second-Generation Systems

Unlike first-generation systems, which are analog, second-generation systems
are digital. The use of digital technology has a number of advantages,
including increased capacity, greater security against fraud, and more
advanced services.
Like first-generation systems, various types of second-generation technology
have been developed. The three most successful variants of secondgeneration
technology are Interim Standard 136 (IS-136) TDMA, IS-95
CDMA, and the Global System for Mobile communications (GSM). Each of
these came about in very different ways.